The invention relates to a method of transmitting a data block of a type from a transmitting device to a receiving device. Furthermore, the invention is related to a transmitting device for transmitting a data block of a type to a receiving device.
Although the physical data connections of modern networks, e.g. the cables, fiber optic cables, get better and better, errors during data transmission cannot be avoided. For this reason, numerous possibilities for error detection and error correction have been developed, e.g. the cyclic redundancy check (CRC). However, the problem of erroneous data blocks still persists.
As an example for these networks, enhanced general packet radio service (EGPRS) networks are exhibited hereinafter. However, this discussion is not limited to this kind of network and one skilled in the art will easily apply the teachings of this document to other networks. In this context, reference is made to the standard ETSI TS 144 060 V7.12.0, Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Mobile Station (MS)-Base Station System (BSS) interface; Radio Link Control/Medium Access Control (RLC/MAC) protocol; (3GPP TS 44.060 version 7.12.0 Release 7), which is herewith incorporated by reference, particularly chapter 9.1.8.2.4, ‘Interpretation of the bitmap’. In particular, also definitions, messages, message flows, and abbreviations, which are used in this application, can be found in this standard.
In this enhanced general packet radio service standard a kind of fast inband signaling has been added recently in addition to the traditional packet downlink acknowledgment (PDAN) and the packet uplink acknowledgement (PUAN). This fast inband signaling contains a short bitmap for positive and negative acknowledgements of radio link control blocks (RLC blocks) and thus speeds up the retransmission of incorrectly received RLC blocks, wherein an RLC block is an example for a data block or a data unit.
This new signaling is called ‘fast acknowledged/not acknowledged reporting’, FANR for short, and will, if used, substantially increase the frequency of acknowledged/negatively acknowledged reporting (ACK/NACK reporting). However, these reports are not as well protected against undetectable decoding errors as the PDAN and PUAN because their CRC is weaker.
Considering a ‘transmitting endpoint’, ‘transmitting network node’, ‘transmitting device’ or simply ‘transmitter’, i.e. one that transmits data blocks and receives acknowledgement data, and a ‘receiving endpoint’, ‘receiving network node’, ‘receiving device’ or simply ‘receiver’, i.e. one that receives data blocks and transmits acknowledgement data (which may be piggy-backed onto other data blocks), a particular risk is that a transmitting endpoint may incorrectly consider a piggy backed acknowledgement (PAN), also referred to as ‘piggy-backed Ack/Nack’, to be included in a data block which it decodes, because an incorrect sequence of bits, which are decoded, causes the CRC check to pass. This may happen for example when a header bit is misinterpreted as PANI (PAN Indicator) field (the PANI field indicates the presence of a PAN field and is arranged in the header of a data block). This is referred to as a ‘false positive’. A ‘false positive’ PAN can also occur when a PAN is included, but when the bit error rate (BER) is too high for successful channel decoding and when the (decoded) CRC nevertheless is correct. Since the payload and the PAN in a radio block can address different mobile stations (MS), a high BER can be expected e.g. for an MS at the cell border if the payload transmission addresses an MS close to the base station and when the transmission uses 32-QAM (quadrature amplitude modulation). The combination of more frequent reporting and weaker protection will lead to more undetected errors which, in many cases, will cause an abnormal temporary block flow release (TBF release). A particular problem is that this release can rely on existing timers which are set to several seconds (e.g. 5 seconds).
Although the CRC size inside the PAN can be increased, e.g. from 6 to 10, to reduce the probability of a ‘false positive’ acknowledgement, the probability never will be zero. In some cases, the CRC is XOR-ed with the temporary flow identity (TFI) for the destination MS. If a different MS, which is multiplexed on the same resources, tries to decode the PAN applying an XOR (Exclusive or Operation) with its (different) TFI, the CRC properties can be weakened. Thus, several error events may occur per hour in a multislot EGPRS transmission.
The result of the reception of a ‘false positive’ may be:
1. That an RLC transmitting endpoint incorrectly believes a block has been acknowledged negatively
2. That an RLC transmitting endpoint incorrectly believes a block has been acknowledged positively
Case 1 causes unnecessary retransmissions.
Case 2 could cause the transmit window to advance further than the receive window. In this case:
                a) the transmitting endpoint will transmit blocks with higher SSNs (starting sequence numbers) than the peer (receiving endpoint) expects, and        b) the transmitting endpoint will not re-transmit blocks which the peer (receiving endpoint) expects.Case b) could eventually end in a TBF deadlock, if one device, i.e. the receiving device, expects retransmissions, but the other device, i.e. the transmitting device, has discarded the block(s) from its memory, in particular its transmit buffer.        
One option is to further increase the CRC length which would reduce the occurrence rate of such errors. However, this would also reduce the number of bits available for payload. It is therefore proposed to take measures on protocol side to minimize the impact of such error events.
The problem of stalled windows (where a transmitting device is hindered from transmitting further blocks, due to waiting for an acknowledgement of earlier transmitted blocks) is already known. The solution is to simply wait for an acknowledgement, which may indicate negative acknowledgements for some data blocks (in which case these are then retransmitted) and/or positive acknowledgements for other data blocks (which may allow the transmitting entity to discard some buffered blocks and possibly advance the transmit window, allowing new blocks to be transmitted for the first time). A drawback of this method is that the transmission of some data blocks is delayed.
However, the inventors' proposal addresses a new problem caused by the relatively high probability that an acknowledgement indication may suffer from a ‘false positive’ detection. This increased ‘false positive’ probability arises from, for example, the specific nature of the fast ACK/NACK reporting scheme used in EGPRS networks, whereby a very small ACK/NACK field is piggy-backed on existing data blocks, and the ACK/NACK field is coded independently of the rest of the block.